Conventionally, a junction box of a photovoltaic power generation module is constructed of an upper case and a lower case, and an electronic circuit is accommodated therein. The upper case of this kind is generally made up of a substantially rectangular parallelepiped member whose lower part is opened or simply made up of a substantially rectangular tabular member (e.g., see Patent Documents 1 and 2).
Substantially rectangular parallelepiped members whose upper part or lower part is opened are used not only for a junction box but also for other industrial parts such as an oil pan of an engine and a waterproof pan for placing a washing machine.
Such substantially rectangular parallelepiped members are required to have a certain degree of strength depending on their target industrial parts. For example, since a junction box may be installed outdoors, its substantially rectangular tabular portion is required to have the strength resistant to a load from the outside (e.g., collision load of flying objects such as hail). Conventionally, various efforts have been made to secure the strength, including increasing the thickness of the tabular portion, modifying the material of the tabular portion, and the like.